Inductively Coupled Plasma Mass Spectrometry (ICPMS) - Overview, Working, And Applications

Scanning Electron Microscopy (SEM)

Discuss about the Inductively Coupled Plasma Mass Spectrometry.

Scanning electron microscope(SEM) uses electrons in forming image of specimens. It offers number of advantages  in comparison with conventional microscope. It has greater scope of field, high resolution and uses electromagnets. Specimens which are closely spaced can be analysed at higher levels of magnification and researcher easily controls magnification. It produces images of exceptional clarity. Hence SEM is a primary tool for researchers in the present day.

An electron gun produces an electron beam which is focussed toward the sample by lenses and electromagnetic fields. The electron beam strikes the sample.  X-rays, electrons are ejected. These are collected by detectors and converted into signals. These signals are transmitted to a monitor and the image  of specimens  is displayed.

Water content from the samples is removed since water vaporises in vacuum condition which exists in the SEM. To protect the users from associated radiation hazard  during the operation of SEM, it is ensured that shielding is not undermined and the levels of exposure  are very less (Scanning Electron Microscope n.d.).

In a transmission electron microscope (TEM) the  basic principle of operation is similar to that of light microscope. The wavelength of light acts as limiting factor in the light microscope. Instead of light,electrons are used in TEM. Due to low wavelength of electrons the resolution of image in a TEM is more than thousand times better in comparison with the light microscope. The high magnifications obtained in TEM has enabled its use in materials, medical and biological research.

Electrons emitted by a light source are focused by electromagnetic lenses into a thin beam. The electron beam is transmitted through the specimen under study. Some electrons undergo scattering and disappear. The scattering depends on the material's  density. The electrons which are not scattered strike a fluorescent screen. A shadow image of the specimen is formed on the screen. The different parts of specimen are displayed at various darkness levels depending on their density. A camera  is used to take photograph of the image or  the operator/ user can directly study the image on the screen(The Transmission Electron Microscope 2016).

Optical emission spectrometry principle: Electrical energy in the nature of spark  is applied between a metal sample and electrode. The atoms in the sample are vaporised  and move to high energy state due to generation of discharge plasma.

The excited ions and atoms which are present in the discharge plasma result in the creation of characteristic emission spectrum of light which is specific to the elements present in the sample. A single element  can generate number of characteristic spectral lines.

Transmission Electron Microscopy (TEM)

Light which is generated by  plasma consists of spectral lines which are produced by   metal sample elements. A diffraction grating is used to split the light and emission spectrum of target elements is  extracted. The intensity of spectrum is dependent on concentration of elements present in metal sample. Photomultiplier tube detectors measure spectrum  intensity of every element after the absence or presence of spectrum is detected. Quantitative and qualitative analysis is performed making use of intensity measurements.

The spectrometer is used for quality control in the manufacture of steel and in processes related to aluminium metallurgy(Principle of Optical Emission Spectrometry 2016)

FTIR(Fourier Transform Infra-red Spectrometer) find application in analysis of food,organic synthesis among other industries. They are combined with chromatography to investigate the mechanisms involved in chemical reactions and  detect volatile substances.

Absorption spectroscopy which utilises infra-red ray radiation is used for determining molecular structures. The molecules of the sample are exposed to infra-red rays. The  molecules assimilate certain wavelengths  of infra-red radiation. The vibration of molecules is displayed in the Infra-red spectrum. The dipole moment of molecules changes. Molecules are transferred from the ground state to an excited state. Total number of absorption peaks are related to vibration modes of molecules. The change in dipole moment and energy level transition is related to absorption peak intensity in the spectrum. The infra-red spectrum is subjected to thorough analysis  using Fourier transform for obtaining  structural information related to the molecule.

FTIR spectrometers have advantages like  higher signal-to-noise ratio corresponding to the spectrum, higher accuracy of wave-number since error is small range,short scan time,wide range of scan,very high resolution,reduction of  interference caused due to stray light.

FTIR spectrometer comprises of source,sample chamber,Michelson interferometer,A/D converter,amplifier,detector and computer. The radiation generated by the source passes through the sample via interferometer and finally reaches the detector. The light beam is split is split by the interferometer. The two light beams travel in different paths to reach the sample. The two beams are later combined by the interferometer into a single beam and is guided to the detector. Here the intensity difference between the two beams is measured as a mathematical function of path difference. The detector produces a time domain interferogram.

The analogue signal generated by the detector is amplified by the amplifier. The amplified signal is converted to digital form. In the computer  the digital signal  is analysed using Fourier transform. The  signal analysed using the Fourier transform is a plot of signal intensity against wave-number(Birkner & Wang 2015).

Optical Emission Spectrometry Principle

Inductively coupled plasma mass spectrometry (ICPMS) finds routine deployment  in diverse fields which include geochemistry, life sciences ,forensic science, archaeology,  environmental science and different industries. ICP MS  is used for detection, identification and reliable quantification of trace elements. The instruments based on ICP MS are robust,reliable and possess very large spectral resolution. They are used for detecting multi-element isotopes. ICP MS possesses properties like high sensitivity,response which is not dependent on compounds present in sample, comparative salt tolerance and high quantification accuracy. They are also used in investigating   health crises  like nerve degeneration caused by  methyl mercury,poisoning of drinking water by arsenic compounds, brain damage caused by lead compounds(Ammann 2007).

An ICP-MS combines high-temperature source containing argon and a mass spectrometer. The source converts atoms in the sample to ions. Later separation of ions  takes place. The spectrometer detects the ions. The sample, in the form of aerosol is exposed to the discharge plasma. The elements in the  aerosol get converted into  atoms which are in gaseous state. Then the atoms are ionised.

Electrostatic lenses focus the ions. The optical system can comprise of one lens or 12 lenses. The ions and the electrostatic lens are positively charged. The lens collimate the beam of ions. Then the ion beam is focussed onto the slit of spectrometer.  The separation of ions in spectrometer is performed based on  mass-to-charge ratio. Quadrupole mass filter is the  commonly used  mass spectrometer.

The ions  are then detected and counted using an appropriate detector. The ions which strike the detector are converted into an electrical signal. The signal is measured and analysed. Information related to number of atoms of particular elements present in the sample is available after analysis which is performed using known calibration standards(Wolf 2013).

X-ray diffraction analysis is a distinctive method for determining  crystallinity of  compound.  A crystal possesses well-defined properties like intrinsic dissolution rate melting point and solubility.  Knowledge  about crystalline structure of compounds is required in controlling the final nature of a product during the  development  stages. Diffractogram is the result of  XRD analysis. It displays intensity as a mathematical function of diffraction angles. A material can be positively identified making use of XRD analysis.

The X-rays which are generated in the system undergo filtration process to create radiation of a single frequency. It is then concentrated using a collimator and focussed towards the sample.  Interaction between sample and incident ray generates  constructive interference along with diffracted ray when the concerned physics law is satisfied.

XRD analysis generates a unique diffraction pattern and provides a novel fingerprint of crystals present in given sample. Interpretation of  diffraction patterns is performed in comparison with standard measurements and reference patterns. The fingerprint enables easy identification of crystalline form present in the sample(X-Ray Diffraction  n.d.).

Reference List

Scanning Electron Microscope  n.d., Purdue University-Radiological and Environmental Management, accessed 26 Nov 2016, https://www.purdue.edu/ehps/rem/rs/sem.htm.

The Transmission Electron Microscope 2016,Nobel Media,accessed 26 Nov 2016,https://www.nobelprize.org/educational/physics/microscopes/tem/.

Principle of Optical Emission Spectrometry 2016,Shimadzu-Analytical and Measuring Instruments,accessed 26 Nov 2016, https://www.shimadzu.com/an/elemental/oes/oes.html

 Birkner,N & Wang,Q 2015. How an FTIR Spectrometer Operates.University of California-Davis,accessed 26 Nov 2016, https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Infrared_Spectroscopy/How_an_FTIR_Spectrometer_Operates.

Ammann AA 2007. Inductively coupled plasma mass spectrometry (ICP MS): a versatile tool. PubMed,accessed 26 Nov 2016,https://www.ncbi.nlm.nih.gov/pubmed/17385793

Wolf,R.E 2013. What is  ICP-MS ?... and more importantly, what can it do?U.S. Geological Survey,accessed 26 Nov 2016,https://crustal.usgs.gov/laboratories/icpms/intro.html.

X-Ray Diffraction – XRD n.d, Particle Analytical,accessed 26 Nov 2016,https://particle.dk/methods-analytical-laboratory/xrd-analysis/